1. Field of the Invention
The present invention relates to apparatus for projecting an image of a pattern formed on a photo mask onto a substrate through a projection optical system. More particularly the present invention relates to an exposure apparatus for use in the optical lithography of semiconductor devices such as IC and VLSI.
2. Description of the Prior Art
Apparatus for optically printing a circuit pattern described on a mask on a wafer by exposure is known and widely used. As a type of the apparatus there has recently been proposed a minifying projection type exposure apparatus generally called stepper. This exposure apparatus is now coming into wide use in the technical field of semiconductor devices.
The stepper uses a projection lens through which an image of a pattern on a mask can be projected on a wafer at a minification of 1/5 or 1/10. The wafer is placed on a stage for two-dimensional movement and exposed to the pattern image. After one exposure the stage is moved a determined distance and then the next exposure is carried out. In this manner, exposure is repeated many times on the wafer while moving the stage at the same and constant pitch after every exposure.
The projection lens used in this type of stepper is required to be bright and have a high resolving power. To satisfy the requirement the lens has a large numerical aperture and a very short depth of focus. The depth of focus is only in the order of several .mu.m although it is variable depending on the type and construction of the lens. Therefore, in projecting the pattern image from the mask onto the wafer it is essential to precisely focus the image on the wafer. To attain the necessary accurate focusing there has been proposed a focus detection apparatus for detecting the position of the wafer surface. An example of such a focus detection apparatus is disclosed in Japanese Patent Application laid open No. 42,205/1981. In this prior art apparatus, an image-forming light beam is obliquely projected on the wafer surface and detection is made for the position at which the reflected beam from the wafer surface is received. The position of the wafer in the direction along the optical axis of the projection lens can be detected by the detection of the reflected beam-receiving position.
To this end, the apparatus is preadjusted in such manner that when the pattern image is correctly focused on the wafer, the reflected beam-receiving position lies just at the origin of a coordinate. Therefore, this type of known detection apparatus needs a very careful adjustment at its manufacture. After the adjustment has exactly been made once, man can focus the pattern image on the wafer simply by moving the wafer up or down in the direction of the optical axis of the projection lens until the beam-receiving point gets in the origin. Thus, man can projects a pattern image always in focus on the wafer.
However, the atmosphere in which the projection optical apparatus is used is variable and the apparatus is sensitive to the change of the atmospheric conditions such as temperature and atmospheric pressure. Although it is very small in amount, the position of focus of the projection lens (the position of the focal plane) shifts in the optical axis direction with the variation of temperature and pressure. This phenomenon is known as focus shift. Due to the focus shift it is difficult to keep the focused relation between the focal plane and the wafer surface. In practice it has been impossible to attain the necessary high accuracy of the alignment (focusing) merely by moving the wafer up or down for alignment with the aid of the oblique incidence type focus detection apparatus according to the prior art. This is an important drawback of the prior art apparatus.